One design for conventional microphones is a condenser microphone. Condenser microphones, also referred to as capacitor microphones, have a diaphragm act as one plate of a capacitor. Vibrations from an acoustic signal (sound) produce changes in the distance between the two plates, thus affecting the capacitance across the plates and/or the voltage across the plates. By measuring one of these, an electrical signal corresponding to the sound can be produced. One particular type of condenser microphones is the electret condenser microphone (ECM). An ECM uses a permanently-charged material, the electret (a dielectric film that has a permanent electric charge), on the top of a back plate.
A developing technology for portable electronic devices involves the application of microelectromechanical systems (MEMS) to microphones. MEMS technology enables the construction of small mechanical components on a substrate, such as a printed circuit board (PCB). MEMS are generally comprised of components 1-100 micrometers (microns) in size (0.001-0.1 mm) and MEMS devices generally range in size from 20 micrometers (0.02 mm) to 1 mm. The standard constructs of classical physics do not always hold true at these size scales. Surface effects such as electrostatics and wetting dominate volume effects such as inertia or thermal mass due to MEMS' large surface area to volume ratio.
FIG. 1 depicts a cross section of the general architecture of a MEMS microphone 100. A diaphragm 102 is disposed in front of a plate 104 and is configured to vibrate freely in response to sound 106. A charged capacitor is formed by two parallel plates where the diaphragm 102 acts as one plate of the capacitor (i.e., the diaphragm 102 is capacitively coupled to the other parallel plate 104). The vibration of the diaphragm 102 results in a change in capacitance of the capacitor, detectable as an electrical signal from the other parallel plate 104. The diaphragm 102 and the plate 104 are held in position by one or more supports 108. As a non-limiting example, the support 108 may enclose or partially define a volume 107 (e.g., a region of air) in front of the diaphragm 102, sometimes referred to as a front volume (located between the incoming acoustic signal and the diaphragm 102, i.e., in “front” of the diaphragm 102 and the plate 104). As a non-limiting example, the support 108 may enclose or partially define a volume 109 (e.g., a region of air) behind the diaphragm 102, sometimes referred to as a back volume (located behind or in “back” of the diaphragm 102). Generally the support 108 is formed from a non-conductive support material.
It should be noted that in other designs a MEMS microphone may have a front plate instead of a back plate. The front plate would be located in “front”. of the diaphragm (e.g., between the diaphragm and the incoming sound). Furthermore, in some designs the front plate or the back plate is porous, having holes through which air can penetrate the plate.
A MEMS microphone offers a number of advantages over an ECM, including advantages in manufacturability, production volume scalability and stability in varying environments, as non-limiting examples. It is often challenging to design an acoustically optimized MEMS microphone package because package design requirements are largely set by the mechanical interfaces of the device in which the MEMS microphone is to be used. For example, the design requirements may depend on how and where the MEMS microphone is integrated in the device.
Generally, there are two basic solutions for implementing a MEMS microphone package in a device: a top port package and a bottom port package. FIG. 2 illustrates a cross section of a top port package 110 for a MEMS microphone 100. The MEMS microphone 100 is disposed on a substrate, such as a PCB 112. Also disposed on the PCB 112 is an application-specific integrated circuit (ASIC) 114. The ASIC 114 generally includes one or more contacts 116 extending along the surface of the PCB 112 or through the PCB 112. These contacts 116 enable the ASIC 114 to connect with other components outside the package 110, such as a processor in the device. Furthermore, these contacts 116 allow the package 110 to be mounted on and/or connected to a larger PCB (e.g., of the device or of another component) within which the package 110 is used and located.
The package 110 also includes a wall 118. The support material from which the wall 118 is formed may be conductive or non-conductive. The wall 118 has a top aperture (opening) 120 in the top of the package 110 for reception of an acoustic signal. The wall 118, PCB 112, support 108 and diaphragm 102 define a region, referred to as a front volume 122, located between the aperture 120 and the diaphragm 102 (i.e., in “front” of the diaphragm 102). The support 108 and the PCB 112 define a region, referred to as a back volume 124, located between the diaphragm 102 and the PCB 116 (i.e., in “back” of the diaphragm 102).
As can be appreciated from FIG. 2, in a top port package 110 the front volume 122 is larger than the back volume 124, leading to undesirable acoustics, including difficulty in achieving an acceptable signal-to-noise ratio (SNR) and unwanted resonance peaks in the frequency response of the useable audio band. Thus, the top port package 110 may have a relatively poor performance level.
FIG. 3 shows a cross section of a bottom port package 130 for a MEMS microphone 100. As compared with the top port package 110 of FIG. 2, the bottom port package 130 has a bottom aperture (opening) 126 in the PCB 112 instead of the wall 118. This leads to reception of an acoustic signal from the bottom of the package 130. Furthermore, note that the diaphragm 102 and plate 104 are reversed such that the plate 104 remains behind the diaphragm 102. As noted above, in other designs a front plate, located in front of the diaphragm 102, may be used.
In the bottom port package 130, the back volume 124 is larger than the front volume 122 leading to improved acoustics (acoustical properties) as compared to the top port package 110. Thus, from an acoustic design perspective, the bottom port package 130 of FIG. 3 is more optimal than the top port package 110 of FIG. 2.
Four alternatives over the basic top port package design 110 of FIG. 2 are discussed below in reference to FIGS. 4-7. FIG. 4 illustrates a cross section of a first alternative top port package 150. The substrate 152 upon which the other components are assembled is designed to have an acoustical cavity 154 connected to the back of the MEMS microphone element 100 by an aperture 156. This effectively enlarges the back cavity 124 and improves the acoustic properties. However, this package 160 is more difficult and more expensive to mass manufacture. Furthermore, and particularly in reference to the bottom port package 130 of FIG. 3, the top port package 150 of FIG. 4 still has a relatively large front volume (front cavity 122) and a comparatively small back volume (back cavity 124).
FIG. 5 illustrates a cross section of a second alternative top port package 160. The top aperture 120 leads to an acoustic channel 162 that extends into an acoustic cavity 164 in the substrate 165. The acoustic cavity 164 connects to an underside of a combined MEMS-ASIC component 166. An interior cavity 167 on the other side of the combined MEMS-ASIC component acts as the back cavity, while the acoustic cavity 164 acts as the front cavity. The second alternative top port package 160 is very difficult to manufacture and has unacceptable acoustical performance.
FIG. 6 illustrates a cross section of a third alternative top port package 170. The top aperture 120 leads to a front cavity 122 that is defined by sealing material 172 and the diaphragm 102. An opening 174 in the support 108 provides an enlarged back cavity 124. While having improved acoustical performance, the third alternative top port package 170 is mechanically unreliable (e.g., fragile and/or susceptible to mechanical forces) and difficult to mass manufacture. In particular, the MEMS microphone 100 and its mechanical connection (adhesion) to the substrate 112 are at risk from mechanical impacts, such as dropping of the package 170. Furthermore, the sealing of the MEMS microphone 100 to the lid is difficult since the sealing acts as a spring, pushing the lid upwards while it should be affixed (e.g., soldered or glued) to the substrate 112.
FIG. 7 illustrates a cross section of a fourth alternative top port package 180. The fourth alternative package 180 resembles an inversion of the bottom port package 130 of FIG. 3 (i.e., turning the package 130 of FIG. 3 upside down). The fourth alternative package 180 includes a top substrate 182 having a top aperture 120 leading to the front cavity 122. The other side of the microphone 190, in conjunction with the wall 118, the support 108, the top substrate 182 and a bottom substrate 184, defines the back cavity 124. Note, however, that it is assumed that the bottom substrate 184 of the package 180 will be mounted on or connected to a larger substrate (e.g., a PCB) of the device. As such, a number of connections (e.g., the five connections 186) are provided to enable the ASIC 114 to communicate with other components of the device. As a non-limiting example, the connections 186 may comprise a series of stacked vias. While providing improved acoustical performance, the fourth alternative top port package 180 is mechanically unreliable (e.g., fragile and/or susceptible to mechanical forces) and difficult to mass manufacture, particularly due to the required connections. Furthermore, the connections 186 are space-consuming and unreliable, and the ASIC must reside on the top substrate 182 or else it would interfere with the membrane of the microphone or significantly increase the front volume (due to its increased height).